The invention relates generally to radio navigation systems and more particularly to systems based on trilateration wherein radio signals transmitted by a plurality of ground- or space-based terminals are received by a user terminal and utilized by the user terminal in calculating its position and orientation.
Trilateration navigation systems are based on the geometrical principle that the position of a point in space is uniquely determined by the distances of the point from three other suitably-positioned reference points whose locations in space are known. Radio navigation systems based on this principle utilize transmitting terminals at the reference points which transmit signals in which a common time reference is embedded. The user terminal, by measuring the time of arrival of the reference terminal signals with respect to the same common time reference, can determine its range to each of the reference transmitting terminals and thereby its position. If the user terminal does not have a clock of sufficient accuracy for independently keeping track of the common time reference, it can, by measuring the time of arrival of signals from four reference transmitters, also keep its clock synchronized to the common time reference of the reference transmitters.
The principle can be extended to the measurement of the attitude of a platform by utilizing receiving ports at a plurality of points on the platform and measuring the locations of these points relative to the reference terminals.
In general, the navigation data desired includes not only position and attitude but also the rates of change of these parameters, i.e. linear velocity and angular velocity. To achieve these goals, not only must the carrier modulation be tracked but also the carrier phase and frequency for each of the received signals.
User receiver systems for trilateration radio navigation systems are comprised of an antenna, a receiver "front end" for selecting and amplifying the frequency band of interest together with "down" converters for translating the radio frequency band of interest to an intermediate frequency band more suitable for subsequent signal processing, and signal processors for extracting the desired information from the received signals. Commonly today, the received signals are translated to baseband where in-phase (I) and quadrature (Q) signal samplers and digitizers followed by digital signal processors are used to extract range and range rate data from the signals.
The extraction of range and range rate data from the I and Q samples of each received signal begins with a correlation process. The I and Q samples are multiplied by reference signal samples, the reference signal being a locally-generated replica of the transmitted signal, and the resulting products are accumulated with prior products to obtain smoothed I and Q values with greatly reduced noise and interference.
The smoothed I and Q samples are further processed to obtain information as to the offset, or misalignment, between the carrier phase, carrier frequency, and modulation phase of a received signal and the corresponding parameters of the corresponding locally-generated reference signal. Two additional functions performed at this stage are estimation of received signal amplitude and recovery of any auxiliary data modulation that may have been impressed upon the carrier. Within the conventional receiver, all of these processing steps are carried out separately for each received signal.
Tracking loops are provided that provide estimated values of carrier phase, carrier frequency, and modulation phase. After processing the current set of smoothed I and Q data, the tracking loops are updated resulting in updated estimates of carrier phase and frequency and modulation phase relative to the local clock in the receiver. At the same time, the auxiliary quantities of signal amplitude and carrier data bits are updated. Prior to the start of the next correlation interval the updated estimates of carrier phase and frequency and modulation phase are used to derive a corrective command signal that is used to adjust the carrier phase and frequency and the modulation phase of the reference signal generator in the correlation processor for the purpose of ensuring adequate signal correlation during the subsequent correlation interval.
An example of the type of navigation system described above is the NAVSTAR global positioning system (GPS) developed by the United States Government. The NAVSTAR system consists of a constellation of 18 to 24 orbiting satellites that transmit pseudo-random ranging signals from which users with appropriate equipment can obtain three-dimensional location, velocity, and timing information anywhere on or near the surface of the earth. Details concerning NAVSTAR/GPS are given in NAVIGATION: Journal of the Institution of Navigation, Volume 25, Number 2, December, 1978 (entire issue).
Conventional navigation receivers process the plurality of received signals independently, Thus, conventional navigation receivers fail to realize their theoretical performance potential by not taking advantage of the collective information contained in the plurality of received signals in the processing of each of the individual signals.